The invention relates to the field of semiconductor laser devices of both the edge emitting and surface emitting types and is particularly concerned with separate confinement heterostructure (SCH) laser devices (LDs).
LDs emitting in the visible spectral region around 650 nm are key elements in the latest professional and consumer products based on Digital Video Discs (DVDs). The most important semiconductor material for such diodes is the AlGaInP alloy system and optical devices based on these materials are already well developed.
The limiting feature of the AlGalnP alloy system for visible laser diode production is the relatively small conduction and valence band offset between the constituent semiconductors in a heterostucture laser. For instance, the maximum conduction band offset obtained in (AlGa).sub.0.5 In.sub.0.5 P is only 270 meV compared to 350 meV in the GaAs/AlGaAs case. Consequently, there is a significant thermally activated electron leakage current in these diodes, leading to high laser threshold currents and poor temperature characteristics. The loss of holes towards the n-type contact, though less significant than electron loss, is also a contributing factor.
A number of approaches have been taken to improve this limitation, notably an increased p-type doping in the p-type cladding region of the LD and the use of so-called Multiple Quantum Barriers (MQBS) near the p-type cladding region (see, for example, D. P. Bour et al, IEEE J. Quant. Elect., vol 30, 2, pp 593-606 (1994)). Both approaches attempt to reduce the electron leakage current by presenting an increased potential barrier to electrons thermally excited out of the active region of the LD, and swept towards the p-type contact by the applied electric field.
Two approaches can be taken to increase the relative population of carriers (electrons and holes) radiatively combining in the active region (and hence contributing to lasing) to those lost by thermal excitation into the p-type contact or recombining non-radiatively in the cladding regions. Inclusion of barrier layers, such as MQBs or electron-reflecting layers, attempt to decrease the leakage current in devices by preventing the loss of electrons (or holes in the case of hole barrier layers).
A second approach is to increase the efficiency of electron capture into the active region of the device. In P. Bhattacharya et al, IEEE J. Quant Elect., vol. 32, 9, pp 1620-1629 (1996), the capture of electrons is promoted by employing a tunnelling region by which high energy injected electrons are cooled before capture into quantum well active regions. This decreases the effect of hot carriers in the wells and also limits the loss of high energy electrons traversing the active region without being captured by the wells, as illustrated in accompanying FIG. 1 which is a schematic diagram illustrating the valence and conduction band energies of a resonant-tunnelling injection LD. In FIG. 1, active region 10 is within an optical guiding region 12 consisting of an n-side guiding region 12a and a p-side guiding region 12b. N-type and p-type cladding regions 14 and 16, respectively, are provided on opposite sides of the active region 10 and guiding regions 12a and 12b. The n-side guiding region 12a is wide and comprises a portion 12c remote from active region 10. The portion 12c has a lower bandgap energy than the region 12b, but there is also provided resonant tunnel barrier region 12d in the n-side guiding region 12a. Electrons from the cladding region 14 are confined in the guiding region 12c by the energy barriers defied by cladding region 14 and the resonant tunnel barrier region 12d. High energy ("hot") electrons cool down in the region 12c by losing energy. Low energy ("cold") electrons in the region 12c are injected into the active region 10 by resonant tunnelling through the quantum confined energy level in region 12d. Holes from the p-type cladding region 16 reach the active region 10 by traversing the p-side guiding region 12b without tunnelling.
R. Kumar et al, Appl. Phys. Lett 68, 26, pp 3704-3706 (1996) disclose the provision of an MQB at the centre of the quantum well active region of an AlGaAs laser in order to promote the capture of injected holes. An MQB in the p-type cladding region is designed to reflect electrons towards the quantum well active region, but this has the unwanted effect of reflecting holes away from the active region. Placing an MQB in the centre of the active region is designed to overcome this problem.
P. M. Smowton et al in Appl. Phys. Lett., vol. 67, pages 1265-1267, 1995 show that an important leakage mechanism for electrons can be via the indirect X-valley of the conduction bands in the p-side guiding and cladding regions of a separate confinement heterostructure (SCH) laser having two Ga.sub.0.41 In.sub.0.59 P quantum wells separated by a barrier, all set in an optical guiding region of (Al.sub.y Ga.sub.1-y).sub.0.51 In.sub.0.49 P (where y is variously 0.3, 0.4 and 0.5), and clad with (Al.sub.0.7 Ga.sub.0.3).sub.0.51 In.sub.0.49 P cladding regions doped with Zn on the p-side and Si on the n-side. However, no proposals are made for mitigating the problems caused by loss of electrons via this mechanism.
Accompanying FIG. 2 shows the band offsets for lattice matched (AlGa)InP. The minimum energy in the conduction band of AlGaInP is a finction of aluminium content, with a crossover from a .GAMMA.-band minimum to an X-band minimum at a concentration of 0.55. The terms .GAMMA.-band and X-band as used herein refer to symmetry points in the Brillouin zone and are standard terms in solid state physics, see for example R. A. Smith, "Semiconductors", (Cambridge University Press, 1978). The terms .GAMMA.-minimum and X-minimum refer to the minimum energy level of the .GAMMA.-band and the X-band, respectively.
Accompanying FIG. 3 illustrates the conduction and valence band profiles of a typical InGaP/AllnGaP multi-quantum well laser showing the relative positions of the .GAMMA.- and X-minima in the heterostructure layers. In FIG. 3, active region 10 has three quantum energy wells formed of Ga.sub.0.5 In.sub.0.5 P set in optical guiding region 12. N-side and p-side guiding regions 12a and 12b are formed of (Al.sub.0.5 Ga.sub.0.5)).sub.0.5 In.sub.0.5 P. N-type and p-type cladding regions 14 and 16 are formed of respectively n- and p-doped (Al.sub.0.5 Ga.sub.0.5)).sub.0.5 In.sub.0.5 P.
Optical transitions giving rise to laser action in the active quantum well region 10 of the laser diode originate from .GAMMA.-electrons in the InGaP quantum wells. FIG. 3 indicates that, in many layers, the X-minimum is of the same or lower energy than the .GAMMA.-minimum. As noted by Smowton et al (supra), a significant percentage of the injected electron population, as well as thermally activated leakage electrons, reside in the X-valley of the AlGaInP cladding, guiding and barrier regions of the laser diode.
As can be seen from FIG. 3, near the p-type cladding region 16, there is a low energy transport path via the X-minima with a lower activation energy for thermal loss of electrons from the wells of the active region to the X-bands than to the corresponding .GAMMA.-bands in the guiding region 12b and cladding region 16.